Flow rate for 1.25 inch hard copper

I live in a Chicago Bungalow. We're adding a moderate (600 sq ft) addition. I'm doing as much of the plumbing and electrical work myself (it's a nice break from the office).

The scope of my addition officially changed this evening. I was planning on leaving the old cast iron, but decided that since I'm replacing 4 of the 7 radiators with underfloor hydronic, I should take this opportunity to remove as much of the old cast iron as possible (headroom problems).

I'm a little concerned that I need to watch the flow rate in the new copper I'll be running between the boiler and the pumps/manifolds. Is there a good online source for max flow rate of 1.25" hard walled copper? I'd really to use one connection to the boiler to the pumps and manifolds

This is a legitimate project. My underground DWV and underground PEX passed on the first try.